1. Field of the Invention
The present invention pertains to information recording and reproduction, and particularly to transfer function compatibility for information reproduction.
2. Related Art and Other Considerations
In the field of information recording and reproduction a head or transducing element is utilized to transduce (e.g., read and write) information relative to a recording medium (e.g., magnetic tape or disk). Typically information recording/reproducing apparatus have a write channel which prepare data for being recorded by the head as signals on the medium, and likewise a read channel which processes signals obtained by the head from the medium prior to transmission to some utilization device, e.g., a host computer or the like. Read channels generally perform various operations relative to the signals acquired from the medium, such as deformatting, for example.
Some read channels assume that the data recorded on the medium has been recorded with a certain transfer function or precode. For example, one type of read channel assumes that a 1/(1xe2x88x92D) precode has been utilized for the recording data. The effect of this 1/(1xe2x88x92D) precode is to convert a data stream where xe2x80x9c1xe2x80x9ds represent transitions into the current waveform in the head. The xe2x80x9cDxe2x80x9d in the 1/(1xe2x88x92D) precode is called the xe2x80x9cdelay operationxe2x80x9d, which for practical purposes of the present invention can refer to a clock period. FIG. 8A shows an example circuit that can be utilized to apply the 1/(1xe2x88x92D) precode to an input data stream.
Other types of read channels assume other types of precode. For example, an EPR4 read channel utilizes a transfer function or precode of 1/(1xe2x88x92D2). FIG. 8B shows an example circuit that can be utilized to apply the 1/(1xe2x88x92D2) precode to an input data stream. By contrast, whereas the read channel having the precoder of FIG. 8A had one flip/flop feed back, the precoder for the 1(1xe2x88x92D2) transfer function has two flip/flops in the feed back. Whereas in the 1/(1xe2x88x92D) precoder energy in the precoded data is maximized at the clock rate, in the 1/(1xe2x88x92D2) precoder the energy is zero at the clock rate and maximized at just under on half of the clock rate.
Because of a difference in preceding, data recorded on a medium using a first transfer function (e.g., the 1/(1xe2x88x92D) transfer precode) cannot normally be read back by a read channel that uses a second transfer function (e.g., the 1/(1xe2x88x92D2) precode). This is problematic when attempting to read, using apparatus with the second type read channel, medium having data recorded using the first transfer function. Such problems can arise, for example, when a first generation recording/reproducing device utilized the first transfer function and a later generation recording/reproducing device utilizes a read channel having the second transfer function.
It is feasible to design a recording/reproducing device with multiple read channels for handling respective multiple transfer functions, and thereby provide some measure of assurance for reading an otherwise incompatible medium. However, the cost of providing (and, in operation, administering) multiple read channels is objectionable.
What is needed, therefore, and an object of the present invention, is a technique for reading a first-precoded data recording on a medium using a second-precode read channel.
Method and apparatus are provided for enabling a reproducing apparatus, having a read channel which utilizes a second transfer function, to accurately recover data which has been recorded on a medium with a first transfer function. A compatibility circuit derives a data rate from the medium, and causes the read channel to sample the data steam acquired from the medium at a multiple of the data rate, thereby creating a multiply-sampled data stream. Further, the compatibility circuit manipulates the multiply-sampled data stream to generate a deformatted read data stream. In particular, a deformatter of the compatibility circuit selects only some bits of the multiply-sampled data stream to generate a modified data stream, and then applies a reverse of the first transfer function to generate a deformatted read data stream.
In an illustrated embodiment, the compatibility circuit comprises a phase lock-loop which derives the data rate from the medium and which causes the read channel to sample the data steam acquired from the medium at the multiple of the data rate. The deformatter comprises both a bit selector (which selects only some bits of the multiply-sampled data stream to generate the modified data stream), and a first transfer function reversal unit (which applies a reverse of the first transfer function to generate the deformatted read data stream).
Thus, the compatibility circuit samples the data stream as if the data stream were recorded using the second transfer function, but with the sampling occurring at the multiple of the data rate to create the multiply-sampled data stream. The compatibility circuit does not utilize the entire multiply-sampled data stream for user data recovery, but rather only selected bits of the modified data stream. Preferably, the selected bits of the modified data stream are alternating bits thereof. One example employment of the invention is when the first transfer function involves a 1/(1xe2x88x92D) precode and the second transfer function involves a 1/(1xe2x88x92D2) precode, in which case the data rate multiple is two. Moreover, the invention provides an enhanced error detection capability by checking the output data by comparing the selected bits of the multiply-sampled data stream with non-selected bits, and detecting an error if the selected bits do not agree with the non-selected bits.